1. Field of the Invention
The present invention concerns a vertical-cavity surface-emitting laser diode (hereinafter referred to as VCSEL) and, it specifically relates to a protective structure for protecting the VCSEL against electrostatic damage.
2. Description of the Related Art
The VCSEL has excellent features in that it has low threshold current, small power consumption and can obtain a circular light spot easily, and light sources can be arranged as a two-dimensional array, and has been expected for use as a light source in optical communication equipment or electronic equipment.
Like other semiconductor apparatuses, the VCSEL is sometimes exposed to a high voltage by static electricity curing handling of circuit substrates, etc. If electrostatic discharge (hereinafter referred to as ESD) should occur inside the device, since a large spike current flows instantaneously, this destroys or deteriorates the device causing a failure of not being capable of conducting normal operation. Several reports have been made for coping with such problems.
In Japanese Published Unexamined Patent Application No. 11-112026, a protective device is provided separately from a light emitting device considering that the withstand voltage is as low as 50 V in the reverse direction and 150 V in the forward direction in semiconductor light emitting devices, particularly, gallium nitride compound semiconductors. As the protective device, a Zener diode or a transistor is used, for example, thereby short-circuiting the reverse voltage or a forward voltage higher than the working voltage possibly applied to the light emitting device.
In U.S. Pat. No. 6,185,240B1, a diode as a countermeasure for ESD and the VCSEL are integrated to form both of them on one identical chip. A trench is formed on the substrate to define a diode region. The diode is, preferably, a p-i-n diode having a breakdown voltage of from 10 to 20 V.
The Japanese Published Unexamined Patent Application No. 2003-110152 concerns an optical semiconductor apparatus with an improved electrostatic withstanding voltage for a light emitting device in which current flows to a light emitting device when a forward voltage is applied and current flows not only to the light emitting device but also to a semiconductor substrate when an over voltage is applied in the forward direction and current flows to the semiconductor substrate when a reverse voltage is applied to prevent damage to the light emitting device, without using a Zener diode but determining a predetermined value in the relation between the resistance of the semiconductor substrate and the light emitting device.
Further, Bobby M. Hawkings, et al. Reliability of Various Size Oxide Aperture VCSELs, Honeywell, 2000 is a study report on the reliability of a selective oxidation type VCSEL and describes a relation between the breakdown voltage due to ESD and oxidized aperture. In this report, ESD damage is tested by a human body model according to US MIL standards, and an oxidized aperture size of from 5 to 20 μm is used as a sample. When a pulse voltage in the forward direction or the reverse direction is applied to VCSEL and the optical output changes by −2 dB, it is defined as damage or failure. FIG. 9 of the Bobby M. Hawkings, et al. Reliability of Various Size Oxide Aperture VCSELs, Honeywell, 2000 shows the result of the ESD damage test. According to the result, it is considered that ESD damage is a function of an oxidized aperture diameter or area and the ESD breakdown voltage increases as the oxidized aperture diameter increases.
However, the existent protective device for the VCSEL has the following problems. Since the protective device is disposed separately from the light emitting device in Japanese Published Unexamined Patent Application No. 11-112026, in a case of handling the light emitting device as a single component, the ESD countermeasure is still insufficient. Further, this increases the number of device constituting the laser apparatus to increase the cost.
Integration of VCSEL and the protective diode on one chip in U.S. Pat. No. 6,185,240B1 is desirable as a countermeasure for ESD during handling but plural trenches have to be formed when the protective diode is formed on the substrate, which complicates the steps and cannot always form the diode easily. Further, in the Japanese Published Unexamined Patent Application No. 2003-110152, the resistance of the semiconductor substrate has to be controlled to a specified value in relation with the light emission device, and the step is not always easy.
Further, while Bobby M. Hawkings, et al. Reliability of Various Size Oxide Aperture VCSELs, Honeywell, 2000 shows that the ESD withstand voltage increases in proportion with the oxidized aperture diameter, a desired basic laser characteristics cannot be obtained by merely increasing the oxidized aperture diameter. Particularly, in a single mode VCSEL, the oxidized aperture diameter is tended to be decreased, which inevitably lowers ESD withstand voltage.